1. Field of the Invention
The present invention relates to a method of achieving enhanced adhesion between a metal and an organic dielectric material, whenever the metal is deposited onto the organic layer by conventional vapor deposition techniques. Specifically, the invention relates to the direct use of a layer of organic dielectric material, exhibiting characteristic surface morphology and topography, for the purposes of metal vapor deposition without any intermediate surface preparatory step of cleaning and etching the organic layer. Quite specifically, the invention concerns the vapor deposition of copper, nickel, chromium or any other metal amenable to vapor deposition onto a specially prepared polymer surface, resulting in an adhesion that is a significant improvement over that observed in vapor deposition on a conventionally prepared polymer surface.
2. Description of the Prior Art
Metallic coatings on polymer surfaces have diverse applications ranging from lustrous finishes on plastic parts and improved solder adhesion on printed circuit boards, to applications in microelectronics, wherein, a polyimide (PI) layer is circuited using Copper lines. In the prior art, the techniques employed to deposit metal onto a polymer surface include evaporation, sputtering, dip coating in colloidal suspensions, electroplating and electroless plating. Typically, a metal layer has no inherent affinity with the inert polymer surface and hence, techniques to create and enhance such an affinity between the metal and the polymer surface are crucial to the integrity of this interface.
Methods of enhancing such an affinity include mechanical and chemical abrasion techniques. Mechanical abrasion techniques suffer the disadvantage of non-uniformity, and have long since been superseded by chemical abrasion methods. This is especially true in advanced applications such as high end microelectronics technology.
Chemical abrasion techniques typically involve etching using an alkaline bath. A smooth organic dielectric material is etched by NaOH or KOH solutions. A typical electroless plating technique involves roughening the polymer surface using etchants and subsequent catalytic seeding and metal plating. U.S. Pat. No. 5,310,580 (O'Sullivan et al.) discloses an improvement upon this electroless plating method by using an etchant on a spontaneously pre-roughened polyimide surface which results in an overall enhancement of the metal-polymer adhesion. U.S. Pat. No. 4,975,327 (Somasiri et al.) disclose a variant chemical abrasion procedure, wherein a polyimide film is coated with a chromium sublayer, and heated to give the oxide. Subsequent deposition of a metal by vapor deposition or electroless plating methods results in an enhanced adhesion. The hydrophilic asperities on the metallic oxide sublayer make it more receptive to subsequent metal deposition.
The use of a reactive chromium sublayer is a common method of achieving adhesion between vapor deposited metal and a polymer film substrate. Typically, a clean polymer surface is sputtered with chromium in the presence of oxygen. This results in a primed surface containing a layer of chromium oxide covalently bonded to the surface. Subsequent sputtering of a metal leads to an enhanced adhesion. The chief drawbacks of this method are: (a) the susceptibility of the chromium layer to hydrolysis, and a resultant weakening of the adhesion over a period of time and (b) an expensive plasma cleaning step using Ar ion sputtering or reactive ion (e.g., oxygen ion) bombardment. For example, in microelectronics, copper is deposited onto a Cr.sup.o primed surface of polyimide. The presence of Cr oxides at the Cu/PI interface affects the bond strength as measured by peel test measurements performed to determine the strength of adhesion.
Consequently, the best solution would be to achieve good adhesion of a conducting metal like copper without the use of any reactive metal sublayer like chromium.
The present invention relates to such a method of improving adhesion between vapor deposited metal and an organic polymer substrate without any special cleaning and surface modification steps that are usually employed.